The organization of data items, such as such as messages, calendar events, tasks, memos, documents, and so forth, is generally driven by information or characteristics that are inherent or expressly present in the data items themselves. Calendar events and tasks, for example, can be sorted according to their associated dates, and events and tasks can be retrieved for a given date can be retrieved on a daily basis. Searches of documents can be executed based on keywords included in the content of the document, or embedded in metadata accompanying the document. In some cases, the characteristics that are inherent in a data item are prescribed by the user: for example, email messages can be filed in user-defined folders according to user-defined labels, and metadata can be added to a document or other data item by the user.
Methods of organization and accessing organized data items are usually constrained to user interface mechanisms supporting text or visual content, since text and graphics are a primary modes of communication between the user and the computing device used to manage the data items. Names of folders and labels are typically defined using text, although in some instances a different informational element (a color or an icon, or both) might be used to distinguish folders and labels (a flag associated with an email in an inbox listing may signify a message that the user needs to reply to; data items in the folder represented by the blue folder icon might be work-related items, while those data items represented by the purple folder icon are personal). In any case, for the organization scheme to be effective, the user must remember the relationship between the text label, color, or icon, and the content of the associated data items.
Visual or textual indicators, though, are not the only means by which humans can organize information or recall associations between data items or even physical objects. For example, spatial relationships between objects (virtual or physical), and spatial relationships between objects and the user, can also be used for organization: urgent matters closer to the user (on a desk), and less urgent matters some distance away (on a side table); incoming documents and documents in a pile on the left, closer to the user (a physical inbox), and outgoing documents in a pile on the right, closer to the door (an outbox); lettermail to be posted on the front hall table, next to the user's house keys, so she will remember to bring the mail with her on her way to work. Some spatial relationships between digital items—data items, such as the aforementioned calendar events, messages, and tasks—can be represented in this manner, but typically this is implemented in a two-dimensional plan view on a computing device display, which limits the ability to represent data items in a truly three-dimensional spatial relationship, or according to a “nearer-farther” scheme.